Oscillator drift compensation device



April c. N. KIMBALL 2,280,527

' OSCILLATOR DRIFT COMPENSATION DEVICE 7 Filed Sept. '7, 1940 70 OTHERTl/NING TOI-Z CONDENSERS 0E7:

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c0MPE-sAT0R {9 4 o g v osc. f 5 :1 -ll- 3 P05/7'll/E 2 2 b L TEMPERATURE $1 6 2 COEFFICIENT ,6 I2 E NEGATIVE Gm NEGATIVE TEMPERATURE E COEFFICIENT POSITIVE To 050. TANK TEMPERATURE E'UEPFM/ENT Pas/Tl vE TEMPERATURE COEFFICIENT Fig. 3 ,l8

+ R QB R lSIEMPEF-{ATURE DEPENDENT 50 J, f T 52; f 53 I 31 5: Rf

ATTORNEY i'ateniesi Apr. 21, 1%42 OSCILLATOR DRIFT'COMPENSATION DEVICE Charles N. Kimball, Jackson Heights, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 7, 1940, Serial No. 355,714

9 Claims. ((31. 250-36) wherein the compensation device comprises an 10 electronic device constructed and arranged to simulate across the tank circuit a reactive effect capable of compensating for the reactive effect causing the frequency drift.

Another important object of this invention is to improve frequency drift compensation devices, and the latter essentially comprising a tube having a mutual conductance of a predetermined polarity sign, a phase shift network being associated with the input and output electrodes of the tube to produce between the output electrodes a capacity efi'ect whose sign of temperature co-' efiicient is the same as, or reverse of,.thesign of temperature coefficient of a capacity included in said phase shift network, but whose magnitude of temperature coefiicient is altered by the capacitive multiplying action of the control tube.

Another object of the invention is to provide a capacity having a negative temperature coeflicient of arbitrary value by utilizing a tube having a negative mutual conductance to provide between its output electrodes the said negative coeflicient, a phase shift network employing a capacity of positive temperature coeflicient being arranged in the tube circuits to provide a phase quadrature relation between the input and output electrode potentials of the tube.

Still another object of my invention is to provide a positive mutual conductance tube with a phase shift network embodying a positive temperature coefiicient condenser, and there being produced between the tube output electrodes a reactive efiect having a positive temperature coefllcient.

Yet other objects of my invention are to improve generally the efliciency and reliability of frequency drift compensation devices, and more especially to provide frequency drift reducing networks, adapted for ultra-high frequency oscillation circuits, which are economically manufac- 0 tured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and 5.3

method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may. be carried into effect.

In the drawing:

Fig. 1 shows an oscillator circuit embodying the invention,

Fig. 2 illustrates a modification,

Fig. 3 shows another modification.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, there is shown a tube I which may be of any wellknown type. The tube is included in the tunable local oscillator network of a radio receiver of the superheterodyne type adapted to receive ultrahigh carrier frequencies. For example, such carrier frequencies may be included in the frequency modulation (FM) band of approximately 40 to megacycles (me). Of course, the present invention is not restricted to use in a radio receiver. Generally speaking it is directed to compensation of frequency drift in any oscillation circuit. Hence, by way of illustration, let tube I be provided with a cathode 2,,wa control grid 3 and a plate 4, and assume that coil 5 and variable condenser 6 provide the usual tunable tank circuit connected between the grid and cathode. The usual grid leak resistor I, shunted by the capacity 8, is inserted in the grid side of the tank circuit.

The plate is connected to the customary positive voltage source through a feedback coil 9 coupled reactively to tank coil 5. Where the circuit is ,used as the local oscillator of a superheterodyne receiver, the locally-produced oscillations may be taken off from the grid and applied to the first detector, or mixer, tube.

Now it is found that during the first 30 to minutes of warm-up time frequency drift occurs in the tank circuit 5-6 due to changes in the values of the frequency-determining reactive elements produced by the thermal effects. In most practical cases the total change in thereactive elements with temperature may be denoted by changes in the capacity of variable condenser 6. Usually such drift has been corrected by paralleling the tuning capacitance with a small condenser known to have an appropriate value of negative temperature coefficient, it being assumed, of course, that the capacitance of the circuit ha a positive temperature coefficient.

According to the present invention a highly flexible and efiicient electronic device is used in place of a physical shunt condenser. A tube is utilized in such a manner that its plate to cathode impedance is essentially a capacitative load across the tank circuit, or a portion thereof. In Fig. 1 the reactance tube is designated by the numeral Ill, and it may be provided with a cathode I I, an input grid l2, an auxiliary grid l3 and a plate I 4. The plate is fed with a proper positive voltage through aradiofrequency choke, while a phase shift network comprising condenser l5 and a series resistor l6 are connected between the plate It and the grounded cathode. The negative biasing source I1 is connected between the lower end of resistor I6 and ground.

The grid I2 is connected tothe junction of condenser l5 and resistor l8. Hence, the grid l2 has a fixed negative bias relative to its cathode. The plate Il may be coupled to any desired point along coil 5 by the condenser l8, and an adjustable tap I! may be used to provide the simulated capacitance 20 across any desired portion of the tank coil 5. The condenser l5 may have any arbitrary temperature coefilcient in this form of the invention. The temperature coefllcient (K) may be positive or negative, but not zero. By way of illustration, let it be assumed that the tuning capacitance of the tank circuit has a positive temperature coeflicient. It is desirable, then, that the simulated capacitance 10 have a negative temperature coeflicient so as to compensate for the capacity change with temperature in the tank circuit. This is readily accomplished by choosing a condenser II whose K is positive, and utilizing for tube I one having a negative mutual conductance (-gm). The latter is simply secured by applying a substantially higher positive voltage to screen l3 than to plate I 4. Those skilled in the art are fully aware .of the many ways of producing a gm tube.

It can be shown that the net change in total capacitance across the tank circuit 58 due to temperature is:

Hence, if K: is positive (value of C2 increasing with temperature) gm must be negative, and vice versa.

The advantage of the present arrangement resides in the control which is afiorded over the effective value of K2. This can be changed merely by adjustment of the 91.1 of tube l0. Thus, eifectively,

C' g R If no control tube 10 were used the value of K in the shunt correction condenser would have to be equal in magnitude, and opposite in sign, to

the value of K in the drifting tuning capacitance. This is of importance to the set designer since it is often difficult to secure physical condensers of just the desired magnitude and sign of temperature coeillcient. The present invention enables the designer to choose at random a condenser I 5 of any small value, and of any sign for K, and by proper adjustment of tube It is able to magnify or diminish the value of IE to the desired value, and even properly adjust the sign of K.

The invention is not limited to use of a negative gm tube at III, since the control tube may be a positive 911. tube. In general, the sign of the resultant capacity temperature K as seen at the control tube plate is determined by the quotient of gm and the phase shifter condenser K. Hence, if either gm or K1; is negative, then K20 is negative. If both K and am are positive or negative at the same time, K is positive. Thus, Fig. 2 shows tube Ill of the high plate impedance type, as a pentode, while the phase shifting network consists of condenser l5 whose K is positive and series resistor I6. The input grid I2 is connected to the junction of I5 and I B. The reactive eifect III in this case is a capacitance having a positive K. The cathode lead may include an adjustable resistor 30, properly by-passed, to permit initial adjustment of the negative bias of grid I2 thereby to control the gain of tube ll. As in the case of Fig. 1, the resistor It should have. a sufilciently small magnitude to cause substantially a 90 degrees phase shift between the plate potential and input grid potential. Under these conditions the plate to cathode impedance of tube II (or It) presents a capacitance eifect across the tank circuit. Those skilled in the art are fully aware of this phenomenon. It can readily be shown that the plate to cathode impedance has a reactance equal to that which would be caused by a condenser whose magnitude and sign depends upon the magnitude and sign of the tube mutual conductance and on the value of the shifting network resistor. As the ambient temperature increases the magnitude of the tuning capacitance varies. Automatically the magnitude of capacity I! varies and by virtue of the multiplying effect of tube III is able to produce a change in simulated capacitance 20 to an extent, and in such a direction, that the tuning capacitance change is accurately compensated for. This invention can readily be utilized for vention. In this embodiment the phase shifter condenser I! has a substantially zero K. The cathode of tube II, which may be a pentode, is grounded. The grid I2 is connected to a nega tive potential point on voltage divider resistance through a path comprising resistor It, lead 5| and tap 52. The condenser 53 bypasses the resistor It to ground. Resistor 5| may include a bi-metailic or other temperature-dependent element so that the control tube grid bias is caused to change in a manner such that the frequency drift is maintained at a low value. The resistor 5| is connected across a direct current voltage supply source of negative polarity say about 20 volts. Tap 52 may be applied to a point which is about -3 volts relative to ground- In the arrangement of Fig. 3 the an of tube Ill varies with temperature, while maintaining Cm constant. Thus, K1: is zero, and the control action would be effected by changes in am. The latter effect is produced by causing the tube bias to vary with temperature. This may be done by employing a special construction for divider 50. The divider resistor 50 is divided into two sections R1 and R2. One section has constant resistance not varying with temperature. The other section has a resistance varying with temperature. Since in most pentodes gm varies approximately linearly with bias it follows that the effective K of the tube is the K of the resistor which varies with temperature in the divider. Thus, if it is desired to have gm decrease as the temperature rises, which would cause the net output capacitance of the control tube to fall as the temperature rises, that is, the effect of a negative K, one can employ elements in the reactance tube grid circuit which have substantially zero change with temperature. There are several possibilities, all of which depend upon the fact that R: is substantially greater than R1. This permits the assumption that I=E/Rz. The bias circuit may then be considered as a "constant current circuit. Either R1 or R2 may be variable with temperature, although both may be variable if their temperature coefficients differ sufficiently to provide a reasonable relative resistance variation with temperature.

R1 and/or R2 may be obtained in either positive or negative K commercially. For example, wire wound resistors (copper, nickel etc.) have positive K. Carbon has a negative K. Based on an assumption of R2 being greater than R1 the following combinations are possible:

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but I that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a resonant circuit adapted to vary in frequency by virtue of temperature effect on the circuit reactance, an electron discharge tube having at least a cathode and two cold electrodes, means applying alternating voltage across said circuit between said cathode and one of said cold electrodes, a phase shifter connected with said one cold electrode for developing from said voltage a second voltage in phase quadrature therewith, means applying the second voltage between the second cold electrode and cathode, and said phase shifter including a reactive element having a temperature coeflicient of predetermined sign so related to the mutual conductance of said tube that a reactive eifect is produced between said one electrode and cathode which has a temperature coeflicient that effectively compensates for said frequency variation.

2. In combination with a resonant circuit adapted to vary in frequency by virtue of temperature effect on the circuit reactance, an electron discharge tube having at least a cathode and two cold electrodes, means applying alternating voltage across said circuit between said cathode and one of said cold electrodes, a phase shifter comprising a condenser in series with a resistor connected with said one cold electrode for developing from said voltage a second voltage in phase quadrature therewith, means applying the second voltage between the second cold electrode and cathode, and said phase shifter condenser having a temperature coefiicient so re-' lated to the mutual conductance of said tube that a reactive effect is produced between said one electrode and cathode which effectively compensates for said frequency variation.

3. In combination with a resonant circuit adapted to vary in frequency by virtue of temperature effect on the circuit reactance, an electron discharge tube having at least a cathode and two cold electrodes, means applying alternating voltage across said circuit between said cathode and one of said cold electrodes, a phase shifter connected with said one cold electrode for developing from said voltage a second voltage in phase quadrature therewith, means applying the second voltage between the second cold electrode and cathode, and said phase shifter being so constructed and related to the mutual conductance of said tube that a. reactive effect is produced between said one electrode and cathode which effectively compensates for said frequency variation, said phase shifter comprising a condenser and resistor in series, said condenser having a temperature coeflicient of a positive sign, said mutual conductance being negative, and said reactive effect being of a negative temperature coefficient.

4. In an oscillator network having a tube provided with a tunable oscillation tank circuit whose tuning capacitance has a temperature c0- efficient of a predetermined sign, a device for producing a capacitance having a temperature coeflicient of the opposite sign, said device comprising a tube having input and output electrodes, said output electrodes being coupled to said tank circuit, a phase shifter network connected to the output electrodes for developing an alternating voltage in substantially phase quadrature with the alternating voltage applied to the output electrodes from the tank circuit, means for applying said developed voltage to the input electrodes, said shifter including a capacitance having a temperature coeflicient of the same sign as said tuning capacitance coeflicient, and said last tube having a mutual conductance whose sign is such that there is developed between said output electrodes a capacitance whose temperature coefficient is opposite in sign to said tuning capacitance coefficient.

5. In an oscillator network having a tube provided with a tunable oscillation tank circuit whose tuning capacitance has a temperature coeflicient of a predetermined sign, a device for producing a capacitance havmg a temperature coefiicient of the opposite sign, said device comprising a tube having input and output electrodes, said output electrodes being coupled to said tank circuit, a phase shifter network connected to the output electrodes for developing an alternating voltage in substantially phase quadrature with the alternating voltage applied to the outv put electrodes from the tank circuit, means for applying said developed voltage to the input electrodes, said shifter including a capacitance having a temperature coemcient of the same sign as said tuning capacitance coeilicient, and said last tube having a mutual conductance whoa sign is such that there is developed between said output electrodes a capacitance whose temperature coefiicient is oppomte in sign to said tuning capacitance coemcient said latter coefficient being positive in sign.

6. In an oscillator network having a tube provided with a tunable oscillation tank circuit whose tuning capacitance has a temperature coeflicient of a predetermined sign, a device for producing a capacitance having a temperature coefficient of the opposite Sign, said device comprising a tube having input and output electrodes, said output electrodes being coupled to said tank circuit, a phase shifter network connected to the output electrodes for developing an alternating voltage in substantially phase quadrature with the alternating voltage applied to the: output electrodes from the tank circuit, means for applying said developed voltage to the input electrodes. said shifter including a capacitance having a temperature coeflicient of the same sign as said timing capacitance coefficient, and said last tube having a mutual conductance whose sign is such that there is developed between said output electrodes a capacitance whose temperature coeiiicient is opposite in sign to said tuning capacitance coefficient said latter coefficient being negative in sign.

7. In an oscillator network having a tube provided with a tunable oscillation tank circuit whose tuning capacitance has a temperature coefficient of a predetermined Sign, a device for producing a capacitance having a temperature coefficient of the oppoflte sign. said device comprising a tube having input and output electrodes said output electrodes being coupled to said tank circuit, a phase shifter network connected to the tance coemcient, said mutual conductance being so chosen that said output electrode capacitance has a coeflcient magnitude variation substantially equal to that of said tuning capacitance coei'iicient change.

8. In an oscillator network having a tube pro I vided with a tunable oscillation tank circuit whose tuning capacitance has a temperature coeilicient of a predetermined sign, a device for producing a capacitance having a temperature coeiiicient of the oppomte sign. B ld device comprising a tube having input and output electrodes, said output electrodes being coupled to said tank circuit, a phase shifter network connected to the output electrodes for developing an alternating voltage in substantially phase quadrature with the alternating voltage applied to the output electrodes from the tank circuit, means for applying said developed voltage to the input elecperature effect on the circuit reactance, an elec-- tron discharge tube having at least a cathode and two cold electrodes, means applying alter-" nating voltage across said circuit between said cathode and one of said cold electrodes, a phase shifter connected with said one cold electrode for developing from said voltage a second voltage in phase quadrature therewith, means applying the second voltage between the second cold electrode and cathode, and said phase shifter being so con structed and related tothe mutual conductance of said tube that a reactive effect is produced between said one electrode and cathode which eii'ecmutual conductance and the condenser temperature coeiiicient.

CHARLES N. KIMBALL. 

